This invention relates to nuclear fuel reprocessing and is particularly concerned with the separation of uranium, plutonium and neptunium.
Most commercial reprocessing plants use the Purex process, in which the spent fuel is dissolved in nitric acid and the dissolved uranium and plutonium are subsequently extracted from the nitric acid solution into an organic phase of tributyl phosphate (TBP) dissolved in an inert hydrocarbon such as odourless kerosene. The organic phase is then subjected to solvent extraction techniques to partition the uranium from the plutonium.
More particularly, the organic phase is subjected to separation of fission products by solvent extraction and typically then to removal of technetium, before the so-called U/Pu split. In the U/Pu split, Pu(IV) is reduced to Pu(III) which is inextractable into the organic phase and therefore follows the aqueous stream while the U remains in the organic stream. Usually, the reducing agent used in the U/Pu split is U(IV). Hydrazine nitrate is normally used to stabilise the U(IV) and Pu(III) against oxidation by, in particular, HNO2 The unit for carrying out the partitioning of the U and Pu in practice comprises a contactor having a multiplicity of stages, for example six stages might be used in a modern centrifugal contactor.
In Purex reprocessing, neptunium valency control can be a significant problem. Neptunium is present in the Purex process as a mixture of three different valence states: Np(IV), (V) and (VI). Np(IV) and (VI) are both extractable into the solvent phase whereas Np(V) is inextractable into this phase. In order to direct Np to raffinate streams, Np is normally stabilised in the (V) oxidation state. This is a complex matter, since not only is it the middle oxidation state of three but Np(V) also undergoes competing reactions, such as disproportionation to Np(IV) and (VI) and is oxidised to Np(VI) by nitric acid. Neptunium control is therefore difficult and efficient neptunium control is a major aim of an advanced reprocessing programme.
After fuel dissolution, Np is likely to be present as a mixture of all three oxidation states. Np(V) will be separated with the aqueous phase at an earlier stage. Np(IV) and (VI) will follow the solvent (containing uranium and plutonium) into the U/Pu split. In the U/Pu split, Np is reduced to Np(IV) which follows the uranium stream into the solvent product. Np is then separated from uranium during the uranium purification cycle.
Commercial reprocessing plants operate multi-cycle processes in which each separation or purification step is performed a plurality of times. It would be desirable to provide a single cycle process, enabling the more efficient reprocessing of fuel.
The present invention provides a spent fuel reprocessing method adapted to reduce Pu to Pu(III) and Np to Np(IV) which includes;
(i) a first step of treating a mixture, containing Pu and Np, and optionally U, with hydroxylamine; and
(ii) a second step of treating the mixture resulting from step (i) with U(IV).
The method is normally performed as part of the U/Pu partition operation. The invention enables the efficient reduction of Pu and Np in the same contactor(s) and obviates the routing of some of the reductant to the subsequent contactor where Np is rejected from the solvent phase. Moreover, the method provides an opportunity to decrease the number of stages in the U/Pu split because the load of U to recover is less than with prior methods which exclusively use U(IV) to reduce Pu and Np.
In preferred methods the mixture is treated first with the hydroxylamine and subsequently with the U(IV). Preferably, the aqueous phase containing the U(IV), is then held at an elevated temperature to allow all or most of the Np to be reduced to Np(IV).
A preferred class of processes are those performed as part of the U/Pu split operation and in which a solvent stream containing U. Pu and Np is contacted with the hydroxylamine and passed through a contactor stage, the aqueous stream from which is then contacted with concentrated U(IV) and supplied to a tank where it is maintained at an elevated temperature for a time sufficient for at least a majority of the Np to be reduced to Np(IV). The resultant liquid is passed through one or more further contactor stages for the re-extraction of U and Np(IV) into the solvent phase.
In another aspect, the invention resides in a spent fuel reprocessing method comprising the partitioning of U and Pu(III) by solvent extraction and the subsequent polishing of the solvent to remove Np therefrom. This method includes the step of recycling solvent from the Np rejection/U recovery (NpA solvent product) operation to the U/Pu partitioning operation. The method enables a reduction in solvent feed and solvent effluent volumes.
In preferred methods of this aspect, the U/Pu split operation is performed in a multi-stage contactor and the NpA solvent product is fed into the solvent stream immediately prior to the final contactor through which the solvent flows.
A further aspect of the invention is a Purex reprocessing plant in which the U/Pu partitioning apparatus includes a criticality safe vessel for temporarily storing the aqueous stream. Preferably the vessel includes means for heating the aqueous stream and/or an inlet for introducing U(IV) into the vessel. The partitioning apparatus may comprise a multi-stage contactor, the vessel being located in the flowpath of the aqueous stream between two contactors.
In a fourth aspect, the invention provides a Purex reprocessing plant in which a conduit is provided between the solvent outlet of the Np rejection/U recovery operation and the U/Pu split apparatus to enable polished solvent to be recycled to the split operation. Preferably, the split apparatus comprises a multi-stage contactor and the conduit enters the solvent flowpath of the partitioning apparatus prior to the final contactor through which the solvent flows.
In the fifth aspect, the invention provides a Purex reprocessing plant in which a conduit is provided between the aqueous product outlet of the Np rejection operation and the U/Pu split apparatus to enable a fraction of the Np rejection aqueous stream to be recycled to the split operation. This conduit normally includes a vessel to allow formohydroxamic acid hydrolysis to go to completion and so produce hydroxylaminc to be used in the U/Pu split. This recycle would minimise the effluent flowrates and, normally, the flowrate of fresh hydroxylamine. The invention therefore includes a spent fuel reprocessing method which includes recycling a fraction of the aqueous stream of the Np rejection operation to the U/Pu split.
In the sixth aspect, the invention provides a Purex reprocessing plant in which a conduit is provided between the solvent stream of the U/Pu split and the Tc rejection or fission product removal operation to enable a fraction of the U/Pu split solvent stream to be recycled to earlier operations. This conduit normally includes a vessel to allow U(IV) oxidation and Pu(III) re-oxidation to go to completion. This recycle would enable a reduction in the solvent feed and minimise the solvent effluent flowrates. The invention therefore includes a spent fuel reprocessing method which includes recycling a fraction of the solvent of the U/Pu split to the Tc rejection or fission product removal operation.